A polarity inverting buck-boost DC-DC converter is one that converts a DC input to a DC output of lower or higher voltage level and opposite polarity with respect to the DC input. Conventionally, a voltage mode converter of this kind includes three regulation and control circuits that act in parallel to maintain an output voltage at the desired level. Each of these circuits contributes to a charge current for a capacitor that is connected to a first input of an operational amplifier the second input of which is connected to a reference voltage source. The output of the operational amplifier is a pulse signal with a variable duty cycle. A first circuit supplies a current contribution in response to the sensed voltage difference between the input and output of the converter. A second circuit supplies a current contribution in response to a voltage difference sensed between the output voltage and a reference voltage and includes a compensation capacitor. The compensation capacitor delays the action of the second circuit. A third circuit also supplies a current contribution in response to a voltage difference sensed between the output voltage and a reference voltage. The third circuit makes a smaller, but faster, contribution to the charge current. This regulation concept is a compromise between fast transient response and stability.
For this second control circuit, a relatively large compensation capacitor usually is needed in continuous mode of the converter. The high capacitance of a large compensation capacitor, however, makes load regulation very bad in discontinuous mode of the converter. As is known, the inductor current increases in the ON phase and decreases in the OFF phase of each clock cycle. In the continuous mode, the inductor current never decreases to zero in the OFF phase. When the average inductor current gets small, and the inductor current reaches zero in the OFF phase and remains zero to the end of the OFF phase. This is called the discontinuous mode. There are solutions that do not allow discontinuous mode, but all these solutions accept reduced efficiency in the discontinuous mode.
Also, it is not possible to implement such a circuit in a polarity inverting buck-boost converter, because only PMOS devices can be applied to a negative voltage and these have to be controlled by negative voltages without any NMOS transistors. There is no acceptable solution for this problem.